Method for improving the heap biooxidation rate of refractory sulfide ore particles that are biooxidized using recycled bioleachate solution

ABSTRACT

A method for improving the heap biooxidation rate of refractory sulfide ore particles that are at least partially biooxidized using a recycled bioleachate off solution is provided. The method includes the steps of biooxidizing a heap of refractory sulfide ore particles with a bioleachate solution; collecting, from the heap, a bioleachate off solution that includes a plurality of inhibitory materials dissolved therein, the concentration of each individual inhibitory material being below its individual inhibitory concentration but the combined concentration of at least two of the inhibitory materials being sufficient to inhibit the biooxidation rate of the refractory sulfide ore particles; conditioning the bioleachate off solution to reduce the inhibitory effect caused by the combined concentration of the at least two inhibitory materials; recycling the reconditioned bioleachate solution to the to the heap; and biooxidizing the refractory sulfide ore particles in the heap with the reconditioned bioleachate solution.

This is a continuation application Ser. No. 08/329,002 filed on Oct. 25,1994 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the recovery of metal values fromrefractory sulfide and refractory carbonaceous sulfide ores. Moreparticularly, the present invention relates to the heap biooxidation ofrefractory sulfide ores using a recycled bioleachate solution.

2. Description of the Prior Art

Gold is one of the rarest metals on earth. Gold ores can be categorizedinto two types: free milling and refractory. Free milling ores are thosethat can be processed by simple gravity techniques or directcyanidation. Refractory ores, on the other hand, are not amenable toconventional cyanidation treatment. Gold bearing deposits are deemedrefractory if they cannot be economically processed using conventionalcyanide leaching techniques because insufficient gold is solubilized.Such ores are often refractory because of their excessive content ofmetallic sulfides (e.g., pyrite and arsenopyrite) and/or organiccarbonaceous matter.

A large number of refractory ores consist of ores with a precious metalsuch as gold occluded in iron sulfide particles. The iron sulfideparticles consist principally of pyrite and arsenopyrite. If the gold,or other precious metal, remains occluded within the sulfide host, evenafter grinding, then the sulfides must be oxidized to liberate theencapsulated precious metal values and make them amenable to a leachingagent (or lixiviant); thus, the sulfide oxidation process reduces therefractory nature of the ore.

A number of processes for oxidizing the sulfide minerals to liberate theprecious metal values are well known in the art. These methods cangenerally be broken down into two types: mill operations and heapoperations. Mill operations are typically expensive processes havinghigh operating and capital costs. As a result, even though the overallrecovery rate is typically higher for mill type processes, milloperations are not applicable to low grade ores, that is ores having agold concentration less than approximately 0.07 oz/ton, and even as lowas 0.02 oz/ton.

Two well known methods of oxidizing sulfides in mill type operations arepressure oxidation in an autoclave and roasting.

Oxidation of sulfides in refractory gold ores can also be accomplishedusing acidophilic, autotrophic microorganisms, such as Thiobacillusferrooxidans, Sulfolobus, Acidianus species and facultative-thermophilicbacteria in a microbial pretreatment. These microorganisms can utilizethe oxidation of sulfide minerals as an energy source during metabolism.During the oxidation process, the foregoing microorganisms oxidize theiron sulfide particles to cause the solubilization of iron as ferriciron, and sulfide, as sulfate ion.

Oxidation of refractory sulfide ores using microorganisms, or, as oftenreferred to, biooxidation, can be accomplished in a mill process or aheap process. Compared to pressure oxidation and roasting, biooxidationprocesses are simpler to operate, require less capital, and have loweroperating costs. Indeed, biooxidation is often chosen as the process foroxidizing sulfide minerals in refractory sulfide ores because it iseconomically favored over other means to oxidize the ore. However,because of the slower oxidation rates associated with microorganismswhen compared to chemical and mechanical means to oxidize sulfiderefractory ores, biooxidation is often the limiting step in the miningprocess.

One mill type biooxidation process involves comminution of the orefollowed by treating a slurry of the ore in a bioreactor wheremicroorganisms can use the finely ground sulfides as an energy source.Such a mill process was used on a commercial scale at the Tonkin Springsmine. However, the mining industry has generally considered the TonkinSprings biooxidation operation a failure. A second mill typebiooxidation process involves separating the gold bearing sulfides fromthe ore using conventional sulfide concentrating technologies, such asflotation, and then oxidizing the sulfides in a bioreactor to alleviatetheir refractory nature. Commercial operations of this type are in usein Africa, South America and Australia.

Biooxidation in a heap process typically entails forming a heap ofrefractory sulfide ore particles and then inoculating the heap with amicroorganism capable of biooxidizing the sulfide minerals in the ore.After biooxidation has come to a desired end point, the heap is drainedand washed out by repeated flushing. The liberated precious metal valuesare now ready to be leached with a suitable lixiviant. Typicallyprecious metal containing ores are leached with cyanide because it isthe most efficient leachant or lixiviant for the recovery of theprecious metal values from the ore. However, if cyanide is used as thelixiviant, the heap must first be neutralized.

Because biooxidation occurs at a low, acidic pH while cyanide processingmust occur at a high, basic pH, heap biooxidation followed byconventional cyanide processing is inherently a two step process. As aresult, processing options utilizing heap biooxidation must separate thetwo steps of the process. This is conventionally done by separating thesteps temporally. For example, in heap biooxidation, the heap is firstbiooxidized and then rinsed, neutralized and treated with cyanide. Toaccomplish this economically and practically, most heap biooxidationoperations use a permanent heap pad in one of several ore on--ore offconfigurations.

Of the various biooxidation processes available, heap biooxidation hasthe lowest operating and capital costs. This makes heap biooxidationprocesses particularly applicable to low grade or waste type ores, thatis ores having a gold (or equivalent precious metal value) concentrationof less than about 0.07 oz/ton. Heap biooxidation, however, has veryslow kinetics compared to mill biooxidation processes. Heap biooxidationcan require many months in order to sufficiently oxidize the sulfideminerals in the ore to permit gold or other precious metal values to berecovered in sufficient quantities by subsequent cyanide leaching forthe process to be considered economical. Heap biooxidation operations,therefore, become limited by the length of time required for sufficientbiooxidation to occur to permit the economical recovery of gold. Thelonger the time required for biooxidation the larger the permanent padfacilities and the larger the necessary capital investment. At minesites where the amount of land suitable for heap pad construction islimited, the size of the permanent pad can become a limiting factor inthe amount of ore processed at the mine and thus the profitability ofthe mine. In such circumstances, rate limiting conditions of thebiooxidation process become even more important.

The rate limiting conditions of the heap biooxidation process includeinoculant access, nutrient access, air or oxygen access, and carbondioxide access, which are required to make the process more efficientand thus an attractive treatment option. Moreover, for biooxidation, theinduction times concerning biooxidants, the growth cycles, the biocideactivities, viability of the bacteria and the like are importantconsiderations because the variables such as accessibility, particlesize, settling, compaction and the like are economically irreversibleonce a heap has been constructed. As a result, heaps cannot be repairedonce formed, except on a limited basis.

The methods disclosed in U.S. Pat. No. 5,246,486, issued Sep. 21, 1993,and U.S. Pat. No. 5,431,717, issued Jul. 11, 1986, by one of the abovenamed, inventors, both of which are hereby incorporated by reference,are directed to increasing the efficiency of the heap biooxidationprocess by ensuring good fluid flow (both gas and liquid) throughout theheap.

Solution inventory and solution management, however, also pose importantrate limiting considerations for heap biooxidation processes. Thesolution drained from the biooxidation heap will be acidic and containbacteria and ferric ions. Therefore, this solution can be usedadvantageously in the agglomeration of new ore or by recycling it backto the top of the heap. However, toxic or inhibitory materials can buildup in this off solution. For example, ferric ions, which are generally auseful aid in pyrite leaching, are inhibitory to bacteria growth whentheir concentration exceeds about 30 g/l. Biocidically active metals canalso build-up in this solution, retarding the biooxidation process.Biocidically active metals that are often found in refractory sulfideores include arsenic, antimony, cadmium, lead, mercury, and molybdenum.Other toxic metals, biooxidation byproducts, dissolved salts andbacterially produced material can also be inhibitory to the biooxidationrate. When these inhibitory materials build up in the off solution to asufficient level, recycling of the off solution becomes detrimental therate at which the biooxidation process proceeds. Indeed, continuedrecycling of an off solution having a sufficient build-up of inhibitorymaterials will stop the biooxidation process altogether.

In the past, to prevent excessive build-up of inhibitory materials inthe bioleachate off solution collected from the heap, mine operationshave simply replaced, or diluted, the effluent from the heap with freshinoculant solution. This is expensive as it increases the consumption offresh water and also increases the need for waste water treatment.

A method is disclosed in U.S. Pat. No. 5,246,486, for removinginhibitory concentrations of arsenic or iron from the heap off solution,which are defined in that reference as concentrations exceeding about 14g/l and 30 g/l, respectively. The method disclosed in this patententails raising the pH of the bioleachate off solution to above 3 sothat the arsenic ions in solution coprecipitate with ferric ions insolution. There are, however, several inadequacies with the processdisclosed in this patent. First, as described above, there are amultitude of potential inhibitory materials that can be leached from theore or that can be formed as a result of the bioleaching process; thus,simply monitoring the arsenic or ferric ion build-up in the bioleachateoff solution will not alleviate the problem of inhibitory concentrationsof other metals or materials from building up in the off solution.Furthermore, the off solution in most instances will not containinhibitory concentrations of any one specific inhibitory material.Nonetheless, the biooxidation process will be retarded from the build-upof a combination of a number of inhibitory materials in the recycled offsolution. Therefore, the combined concentration of at least twoinhibitory materials may be sufficient to inhibit the biooxidation rateof refractory sulfide ore particles in the heap even though theconcentration of no single material is above its inhibitoryconcentration.

Consequently, a need exists in heap biooxidation processes for a methodof removing inhibitory concentrations of a group of inhibitory materialswithin the heap off solution. Such a method would reduce the timerequired for heap biooxidation processes and concomitantly reduce thecapital required for constructing the heap biooxidation facility. Inaddition, such a method would reduce the constriction heap biooxidationtypically places on mine operations.

SUMMARY OF INVENTION

It is an object of the present invention to provide a heap biooxidationprocess of the type described above, wherein the bioleachate solutionmay be recycled with little or no reduction in the biooxidation rate ofthe refractory sulfide ore particles within the heap due to the build-upof an inhibitory concentration of a group of inhibitory materials withinthe heap off solution. To this end a heap biooxidation process isprovided in which a heap of refractory sulfide ore particles isbiooxidized with a bioleachate solution. The bioleachate off solutionfrom the heap is collected. If this solution is inhibitory to thebiooxidation process due to the combined concentration of a group ofinhibitory materials, then the bioleachate off solution is conditionedto reduce the inhibitory effect caused by these materials. Theconditioned bioleachate solution is then recycled to the top of the heapwith little or no reduction in the rate of biooxidation. Alternatively,the conditioned bioleachate solution may be applied to a second heap ofrefractory sulfide particles or used to agglomerate particles ofrefractory sulfide minerals prior to heap formation.

A preferred method of conditioning the bioleachate off solutionaccording to the present invention involves raising the pH of at least aportion of the off solution to a pH within the range of about 5.0 to6.0, preferably to a pH within the range of about 5.5 to 6.0. This couldbe done continuously as a prophylactic measure, or only after it isspecifically determined that the solution is inhibitory. Raising the pHof the bioleachate off solution in this fashion will typicallyprecipitate out the inhibitory materials causing the reduction in thebiooxidation rate. The solid precipitates are then separated from thebioleachate solution and the pH of the solution is lowered to an optimumpH for the biooxidation process. The conditioned bioleachate solution isthen recycled to the heap or used for agglomerating new ore.

The above and other objects, features, and advantages will becomeapparent to those skilled in the art from the following detaileddescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a biooxidation process with a solutionmanagement system according to one embodiment of the present invention;

FIG. 2 is a schematic of prior art "race track" type biooxidationprocess that can be used with the solution management system accordingthe present invention;

FIG. 3 is a graph of the % iron leached from an ore as a function oftime;

FIG. 4 is a graph of the Eh of a bioleachate off solution from arefractory sulfide ore and a corresponding concentrate of the sulfidesfrom the ore as a function of time;

FIG. 5 is a graph illustrating the % Fe leached as a function of timefor an ore in which the bioleachate off solution was recycled withouttreatment;

FIG. 6 is a graph illustrating the % Fe leached as a function of timefrom an ore in which only fresh solution was applied to the ore;

FIG. 7 is a graph illustrating the Eh of various bioleachate solutionsat time 0 and after 24 hours had elapsed;

FIG. 8 is a graph comparing the Eh of an original effluent from an oreto that of an effluent which had been adjusted to a pH of 6.0 and thenreadjusted to a pH of 1.8 without removal of the precipitates formedduring the first pH adjustment;

FIG. 9 is a graph illustrating the extent of pyrite biooxidation as afunction of time for a pilot heap biooxidation process;

FIG. 10 is a graph illustrating the amount of ferrous ion converted toferric ion for various samples;

FIG. 11 is a graph illustrating the mg of ferric ion in varioussolutions as a function of time; and

FIG. 12 is a graph illustrating the amount of ferrous ion converted toferric ion for various samples.

DETAILED DESCRIPTION OF THE INVENTION

According to a first embodiment of the present invention, a method forimproving the heap biooxidation rate of refractory sulfide ore particlesthat are at least partially biooxidized using a recycled bioleachate offsolution is provided. The process comprises the steps of biooxidizing aheap of refractory sulfide ore particles with a bioleachate solution;collecting a bioleachate off solution that includes a plurality ofinhibitory materials dissolved therein from the heap, the concentrationof each individual inhibitory material in the bioleachate off solutionbeing below its individual inhibitory concentration and the combinedconcentration of at least two of the inhibitory materials beingsufficient to inhibit the biooxidation rate of the refractory sulfideore particles in the ore; conditioning the bioleachate off solution toreduce the inhibitory effect caused by the combined concentration of theat least two inhibitory materials; recycling the conditioned bioleachatesolution to the heap; and biooxidizing the refractory sulfide oreparticles in the same heap or a second heap with the conditionedbioleachate solution.

The starting material upon which the present invention can operateinclude refractory sulfide ores and refractory carbonaceous sulfideores. As used herein, therefore, refractory sulfide ore will beunderstood to also encompass refractory carbonaceous sulfide ores.

A schematic illustration of one means of practicing the presentembodiment is provided in FIG. 1.

Referring to FIG. 1, heap 10 is formed of refractory sulfide particleson a reusable leach pad. After heap 10 is biooxidized by a targetamount, heap 10 becomes heap 12, which is allowed to drain. Drained heap12 then becomes wash heap 14. After heap 14 is washed, the refractorysulfide particles in heap 14 are typically removed from the permanentleach pad and the gold recovered in a heap cyanidation process as iswell known in the art.

If the refractory ore being processed is a carbonaceous sulfide ore,then additional process steps may be required following microbialpretreatment to prevent preg-robbing of the aurocyanide complex or otherprecious metal-lixiviant complexes by the native carbonaceous matterupon treatment with a lixiviant.

One known method of heap bioleaching carbonaceous sulfide ores isdisclosed in U.S. Pat. No. 5,127,942, issued Jul. 7, 1992, which ishereby incorporated by reference. According to this method, after theore is subjected to an oxidative bioleach to oxidize the sulfidecomponent of the ore and liberate the precious metal values, the ore isthen inoculated with a bacterial consortium in the presence of nutrientsto promote the growth of the bacterial consortium, the bacterialconsortium being characterized by the property of deactivating thepreg-robbing propensity of the carbonaceous matter in the ore. In otherwords, the bacterial consortium functions as a biological blankingagent. Following treatment with the microbial consortium, whichdeactivates the precious-metal-adsorbing carbon, the ore is then leachedwith an appropriate lixiviant to cause the dissolution of the preciousmetal in the ore as is known in the art.

The first step of the bioleaching process is to obtain refractorysulfide ore particles of an appropriate size for heap leaching. This canbe accomplished by crushing the ore to the desired size range. Therefractory sulfide ore is preferably crushed to a target maximum size inthe range of approximately 1/4 to 1 inch. Appropriate target maximumparticle sizes include 1/4, 3/8, 1/2, 3/4 and 1 inch. If the ore willpass any of these target particle sizes, it should be amenable to heapleaching. The smaller the particle size, however, the greater thesurface area of the sulfide particles in the ore and, of course, thefaster the sulfide particles will be biooxidized. Increased recovery ofthe precious metal values should also result. This, however, must beweighed against the additional cost of crushing the ore to a smallerparticle size. The additional amount of precious metal recovered may notjustify the added cost.

In gold heap leaching, ores are often crushed to about -3/4 inch, whichis a good compromise between reducing rock size to minimize the requiredleaching time and avoiding production of too many fines, which causeslow permeability in the ore heap and hinders the flow of the bioleachatesolution, and subsequently the flow of cyanide solution, percolatingdown through the ore heap. Particle size should be selected so as toachieve the highest rate of biooxidation concomitant with the mosteconomic crushing of the particular ore. Thus for easy-to-crush ores,the size is less, e.g., 1/2 inch to minus ten mesh size but for hard tocrush ores from 1 to 1/4 inches is more typical.

Proper ore crushing and particle size are achieved by means well knownin the art.

Of course if the refractory sulfide ore body being biooxidized isalready an appropriate size for heap bioleaching, no additional crushingis required.

In the event the concentration of acid consumable components of the ore,which are well known in the art, are significant or the ore containsexcessive concentrations of inhibitory materials, an acid pretreatmentof the ore may be necessary to properly condition the ore forbiooxidation. Conditioning of the ore typically includes adjusting thepH of the ore, washing out soluble inhibitory components, and addingmicrobial nutrients followed by aging of the ore.

Conditioning should be initiated as soon as possible. If feasible,conditioning should begin with the ore in situ within the ore body.Subsequent conditioning should be conducted during ore hauling,crushing, agglomeration and/or stacking.

Biooxidation of refractory sulfide ores is especially sensitive toblocked percolation channels by loose clay and fine material because thebacteria need large amounts of air or oxygen to grow and biooxidize theiron sulfide particles in the ore. Air flow is also important todissipate heat generated by the exothermic biooxidation reaction,because excessive heat can kill the growing bacteria in a large, poorlyventilated heap. Accordingly, if the ore is high in fines and loose claymaterial, agglomeration of the ore may be necessary to prevent flowchannels in the heap from becoming plugged.

Alternatively, good fluid flow within the heap can be ensured byremoving the fine and/or clay materials from the refractory sulfide oreprior to heap formation as taught in U.S. Pat. No. 5,431,717, by W.Kohr, which was previously incorporated by reference.

Initial inoculation of the refractory sulfide ore particles withbiooxidizing bacteria is preferably conducted during the agglomerationstep as taught in U.S. Pat. No. 5,246,486, which was incorporated byreference above, or immediately after stacking the ore on the heap.

Although other means of heap construction may be used, conveyor stackingis preferred. Conveyor stacking minimizes compaction of the ore withinthe heap. Other means of heap construction such as end dumping withdozer ripping or top dumping can lead to regions of reduced fluid flowwithin the heap.

Once heap 10 is formed, heap 10 is inoculated with additionalbioleachate solution supplied from tank 18 through line 16 on an asneeded basis. The bioleachate solution supplied through line 16 containsat least one microorganism capable of biooxidizing the refractorysulfide ore particles in heap 10.

A microbial nutrient solution is also applied to heap 10 as required.Nutrient additions are monitored throughout the course of thebiooxidation process and are made based on selected performanceindicators such as the solubilization rate of arsenic or iron in thepyrites or the oxidation rate of sulfides, which can be calculatedtherefrom. Other biooxidation performance indicators that may be usedinclude measuring pH, titratable acidity, and solution Eh.

The following bacteria may be used in the practice of the presentinvention:

Group A. Thiobacillus ferrooxidans; Thiobacillus thiooxidans;Thiobacillus organoparus; Thiobacillus acidophilus;

Group B. Leptospirillum ferrooxidans;

Group C. Sulfobacillus thermosulfidooxidans;

Group D. Sulfolobus acidocaldarius; Sulfolobus BC; Sulfolobussolfataricus and Acidianus brierleyi and the like.

These bacteria are either available from American Type CultureCollection or like culture collections or have been made availablethereto and/or will be made available to the public before the issuanceof this disclosure as a patent.

The Group A and B bacteria are mesophiles, that is the bacteria arecapable of growth at mid-range temperatures (e.g., about 30° C.). GroupC consists of facultative thermophiles because the bacteria are capableof growth in temperature range of about 50° C. to 55° C. Finally, theGroup D bacteria are obligate thermophiles, which can only grow at high(thermophilic) temperatures (e.g., greater than about 50° C.).

It should be noted that the for Group A and B bacteria to remain useful,the temperature of the heap should not exceed about 35° C.; for Group Cbacteria the temperature of the heap should not exceed about 55° C.; andfor Group D bacteria, the temperature of the heap should not exceedabout 80° C.

As is well known in the art, the temperature in a bioleached heap is notuniform and the bacteria are often unable to survive if the temperatureis improperly controlled or if the appropriate bacteria are not used.Consequently, based on a temperature profile of the heap when oxidationof the refractory sulfide ore particles is in its most advanced stageand the sulfide oxidation exotherm is the highest, the heap may bebathed with cooled bioleachant, cooled, recycled bioleachant, or acooled maintenance solution, i.e., a nutrient solution. In addition theheap may be constructed with cooling (and/or heating) provisions.Moreover, the heap may be inoculated with the appropriate bacteria tomeet the temperature limits of the ore. Thus, if the ore is a highsulfide content ore, a thermophilic bacteria should preferably be used.

After the biooxidation reaction has reached an economically defined endpoint, the heap may then be drained and subsequently washed by repeatedflushings with water. The number of wash cycles required are typicallydetermined by a suitable marker element such as iron and the pH of thewash effluent. After wash heap 14 is properly flushed, it is brokenapart, neutralized, and treated in a traditional cyanidation heapleaching process as is well known in the art.

Solution inventory and solution management are an important part of thebiooxidation process. FIG. 1 illustrates a solution management systemaccording to one embodiment of the present invention for the entirebiooxidation, drainage, and wash sequence. From FIG. 1, it can be seenthat according to this embodiment all of the solution values arereutilized. This minimizes the amount of fresh water required by thebiooxidation process.

According to FIG. 1, the bioleachate solution that has percolatedthrough heap 10 is collected and reapplied to the top of heap 10. Thissolution is acidic and contains ferric ion and therefore can be usedadvantageously by recycling it to the top of the heap or by using it foragglomeration of new ore. However, the effluent solution generated earlyin the biooxidation process will also contain significant concentrationsof base and heavy metals, including the components that lead tomicrobial inhibition. As the inhibitory materials build-up in the offsolution, the biooxidation process is retarded.

For example, ferric ions, which are generally a useful aid in pyriteleaching, are inhibitory to bacteria growth when their concentrationexceeds about 30 g/l. Biocidically active metals can also build-up inthis solution, retarding the biooxidation process. Biocidically activemetals that are often found in refractory sulfide ores include arsenic,antimony, cadmium, lead, mercury, molybdenum and silver. Otherinhibitory metals (including copper and aluminum), biooxidationbyproducts, dissolved salts and bacterially produced material can alsobe inhibitory to the biooxidation rate. Anions such as Cl⁻, NO₃ ⁻, andSO₄ ⁻ may also need to be reduced before the solution is recycled backto the heap. When these inhibitory materials build-up in the offsolution to a sufficient level, recycling of the off solution becomesdetrimental to the rate at which the biooxidation process proceeds.Indeed, continued recycling of an off solution having a sufficientbuild-up of inhibitory materials will stop the biooxidation processaltogether.

Further, the normal pH adjustment of the bioleachate off solution to theoptimal pH range for bioleaching is inadequate to remove the inhibitorymaterials from solution. Thus, if the pH of the off solution is merelyadjusted to the optimal range before recycling the solution to the topof the heap, the biooxidation rate will remain suppressed.

Nor is simply monitoring the arsenic or ferric ion build-up in thebioleachate off solution and then treating the off solution when one ofthese compounds are present in excessive concentrations adequate toalleviate the problem of inhibitory concentrations of other metals ormaterials from building up in the off solution. More importantly, theoff solution in most instances will not contain inhibitoryconcentrations of any one specific inhibitory material. But, rather thebiooxidation process will be retarded from the build-up of a combinationof a number of inhibitory materials in the recycled off solution.Therefore, in most instances, the combined concentration of at least twoinhibitory materials will be sufficient to inhibit the biooxidation rateof refractory sulfide ore particles in the heap. Indeed, typically, thebiooxidation rate of the bioleachate off solution will be inhibited dueto the combined concentration of a group of inhibitory materials longbefore the concentration of any one inhibitory material in the groupeven approaches its inhibitory concentration.

As is well known in the art, different bacteria, and different strainsof the same bacteria, have varying sensitivities to the inhibitorymaterials. Thus, the inhibitory concentration of individual inhibitorymaterials will vary with different bacteria and with different strainsof the same bacteria. Indeed some strains will be highly resistant to ametal while others are highly sensitive to it. For this reason, it isuseful to test the bacteria being used in a biooxidation process fortheir sensitivity to metals in the ore and in the effluent or offsolution.

To determine the individual inhibitory concentration of a specificbacteria inoculant, as illustrated in Example 1 below, a simplebiooxidation test can be performed using a bioleachate solutioncontaining a known concentration of the inhibitory material, preferablyin the sulfate form, and a known concentration of bacteria. Theconcentration of the inhibitory material is then increased in a stepwisefashion until an inhibitory effect in the biooxidation rate of thebioleachate is observed. The point at which an inhibitory effect isobserved is the inhibitory concentration for the material. Whether aninhibitory effect is observed is determined by comparing the sample to apositive control.

According to the present invention, the bioleachate off solution istreated in a conditioning circuit 20 to reduce the inhibitory effectcaused by the combined concentration of a group of inhibitory materialsbefore any one specific inhibitory material in the group reaches itsinhibitory concentration. Treatment options for conditioning thebioleachate off solution include lime softening, limestone softening,ion exchange, electrodeposition, iron cementation, reverse osmosis or acombination of these technologies.

In some instances, the concentration of an inhibitory metal may besufficiently high to justify economic recovery of the metal values. Forexample, if the concentration of copper is sufficiently high in thebioleachate off solution, solvent extraction or electrowinning might beemployed to recover this metal.

The preferred method of conditioning the bioleachate off solutionaccording to the present invention is lime or limestone softening. Thisis accomplished by using lime or limestone to raise the pH of thebioleachate off solution to pH of at least 5.0, preferably to a pHwithin the range of about 5.0 to 6.0, and most preferably to a pH withinthe range of about 5.5 to 6.0. The resulting precipitates are thenremoved from the bioleachate off solution. After the precipitates areremoved, the pH of the solution is lowered back to the optimal range of1.2 to 2.6 for biooxidation using concentrated acid or using acid in thewash water 24 and/or drain solution 26. More preferably the pH of thesolution is lowered to the range of 1.7 to 1.9, and most preferably thepH is lowered to a pH of about 1.8. Although lime or limestone is thepreferred means of raising the pH to greater than 5.0, other strongbases can also be used as one of ordinary skill in the art wouldrecognize.

If the treated bioleachate off solution remains too inhibitory afterhaving undergone lime or limestone softening, then the off solution mayrequire further purification by one of the other conditioning techniqueslisted above. Whether another conditioning technique is employed willdepend on whether the incremental improvement in the biooxidation rate,which is achieved by the removal of the additional inhibitory materials,is justified by the added cost of removing the inhibitory materials.

Once the pH of the bioleachate off solution is readjusted to theappropriate pH for biooxidation, conditioning of the bioleachate offsolution is complete and the conditioned solution 22 may be reapplied tothe top of heap 10 to promote additional biooxidation within the heap.Moreover, the biooxidation rate will be higher than that forunconditioned recirculated bioleachate solution, and in some instancesgreater than that by a fresh solution. Alternatively, the conditionedbioleachate solution may also be used to agglomerate ore as it is beingplaced on the heap.

It is economically preferred for the flow rate of the bioleachatesolution through the ore to be as slow as possible. In the case of oresthat require purification or conditioning of the effluent solutionbefore it can be reapplied, the preferred flow rate of the bioleachatethrough the heap is from 0.0005 to 0.003 gpm/ft². In the case of oresthat produce toxic material while being leached, the movement of freshor purified solution through the heap will allow for the growth ofbacteria at least in the upper part of the heap. The bacteria will growin the heap as fast as the elution of the toxic materials will allow.This depth of bacterial penetration may vary, and may be difficult todetermine. However, ferric ions produced by the bacteria in the uppersection of the heap will migrate to the lower part of the heap wherebacterial growth may be inhibited. This will allow biooxidation toproceed even if bacterial growth is not favored. By this method, orethat contains toxic elements or that produces any toxic material as theyoxidize can be biooxidized in a heap by recirculating detoxifiedsolution back to the top of the heap, rather than simply reusing thedrain solution without treatment.

Based on the teachings herein, in many instances, those skilled in theart will recognize from the ore assay alone that the refractory sulfideore they are processing poses a problem with respect to the build-up ofa combination of inhibitory materials in the bioleachate off solution.Based on this knowledge, a decision will be made to simply treat thebioleachate off solution on a continuous basis in a lime or limestonesoftening circuit of the type discussed above before recycling thebioleachate solution. Alternatively, those skilled in the art may decideto treat the bioleachate solution after every pass through the heap inthe lime or limestone softening circuit simply as a prophylacticmeasure. Both of these processes would fall within the teachings of thepresent invention.

The present invention also contemplates processes in which conditioningof the bioleachate off solution is performed in response to anaffirmative determination that the off solution is inhibitory to thebiooxidation process.

Those skilled in the art will immediately recognize that there are anumber of techniques that can be employed for determining whether thebioleachate off solution is inhibited. Many of the techniques may notspecifically determine the concentration of individual inhibitorymaterials. Indeed, it is preferred that techniques be employed whichsimply look at whether the solution is impaired as compared to apositive control. This is because the concentration of inhibitorymaterials found in a bioleachate off solution will change continuouslydepending on such factors as where in the ore body the ore was obtainedand how far the biooxidation process has progressed. Therefore, it wouldbe very difficult, if not impossible, to attempt and determine whether aparticular combination of inhibitory materials at a given concentrationis inhibitory simply by looking at the concentrations of the inhibitorymaterials in the off solution.

On the other hand, by comparing the performance of the off solution to apositive control, it can be easily determined whether the combinedconcentration of inhibitory materials in the off solution is inhibitory.

Further, such testing need not be performed on a continuous basis.Rather, during column or pilot testing of the ore, the typical length oftime that inhibitory concentrations of toxins or inhibitory materialsare leached out of the ore can be determined. With this knowledge, thoseskilled in the art can readily determine how long the biooxidationprocess must proceed before the bioleachate off solution can be safelyrecycled without conditioning to remove inhibitory materials.

Preferably, however, the toxicity of the bioleachate off solution to thebiooxidation microorganism is tested on a continuous basis. In this way,it can be determined whether the bioleachate off solution is inhibitory,the extent it inhibits the biooxidation process, and which treatmentmethods most adequately remove the inhibition. The following two assaytechniques are preferred for determining solution toxicity to thebiooxidation microorganism.

The first, a spectrophotometric activity assay is based on theabsorbance of ferric iron (Fe³⁺) at 304 nm. This procedure is amodification of the method described by Steiner and Lazaroff, AppliedMicrobiology, 28:872-880, 1974, hereby incorporated by reference, todetermine the concentration of ferric ion in a solution. According tothis assay, samples containing a known number of bacteria, the testsolution, and ferrous sulfate are monitored over time (usually 5-20minutes) by measuring their absorbance at 304 nm. These absorbances arecompared over time to a standard curve relating absorbance and ferriciron concentration. As a result, a curve is obtained that describes therate of iron oxidation by the bacteria. The iron oxidation rates ofbacteria in different solutions can then be compared directly, thusgiving an indication of the ability of a solution to inhibit thisactivity. Solutions which slow the rate of iron oxidation by bacteriaare regarded as toxic or inhibitory.

A microtiter plate assay is the second preferred method for measuringthe toxicity of the bioleachate off solution. The spectrophotometricassay is very sensitive to ferric concentration. It only has a workingconcentration range of about 0.1-1000 ppm ferric iron. For samples withhigh ferric iron concentrations, the microtiter plate assay wasdeveloped. As in the previous assay, the test samples include a knownnumber of bacteria, test solution, and ferrous sulfate. In this assay,however, the redox potential (Eh) of the sample is measured over time(usually 24-48 hours). The redox potential is a measure of the ratio offerric to ferrous iron in solution, with higher redox potentialsindicating a high percentage of ferric iron. By knowing the percentageof ferrous iron in the starting material, the percentage of ferrous ironat the end of the assay, and the total amount of iron (combined ferrousand ferric), the milligrams of ferrous iron converted to ferric iron canbe calculated. The activities of the bacteria in different solutions arecompared to a positive control on the basis of the milligrams of ferrousiron converted to ferric iron by the end of the assay, which is when allof the ferrous iron is converted to ferric iron in the positive controlsample.

One advantage of the spectrophotometric and microtiter assays is thatthey permit the rapid determination of the toxicity of specificsubstances.

Using these assays, two types of toxicities to bacteria have beenobserved. They are referred to as "chronic" and "acute". "Acute"toxicity describes the inhibition of ferrous to ferric oxidation whilethe bacteria are in the off solution; "chronic" toxicity describes theability of an off solution to inhibit bacterial oxidation by bacteriawhich have been removed from the solution, washed, and placed into afresh ferrous sulfate medium. The spectrophotometric assay can be usedto test the "chronic" toxicity of solutions, and the "acute" toxicity ofsolutions which do not contain large quantities of ferric iron (i.e.,solutions containing >1000 ppm Fe³⁺). The microtiter assay can be usedto measure both the "acute" and "chronic" toxicities of solutions.

These assays also can be used in combination, to test the "chronic" and"acute" toxicities of a solution. The microtiter plate assay is runfirst, for 24-48 hours, during which time the bacterial iron oxidationin solution is measured ("acute" toxicity test). Then, the bacteria areremoved from the assay plate, pelleted, washed, and resuspended in aferrous sulfate solution. This suspension is monitored at 304 nm for anincrease in ferric iron over time ("chronic" toxicity test). Bycombining these two assays, the ability of a solution to produce animmediate and/or long-lasting toxicity can be determined.

In a preferred variation of the present embodiment, instead ofconditioning the entire volume of off solution, only a portion of theoff solution is treated to remove inhibitory materials. The conditionedportion is then recombined with the unconditioned portion to dilute theinhibitory materials therein before recirculation. This variation isparticularly preferred when using lime or limestone softening as themethod of conditioning since less lime or limestone is required. Afterthe precipitates have been removed from the conditioned off solution,the unconditioned portion of the off solution can be used to lower thepH of the conditioned portion back to the optimum range forbiooxidation.

Preferably about 70% to 85%, more preferably about 70% to 80% of thebioleachate off solution is conditioned by lime or limestone softening.In this way, approximately the same amount of lime or limestone isrequired to raise the pH of the treated volume of solution to thepreferred conditioning pH range of 5.5 to 6.0 as is required to raisethe pH of the entire bioleachate off solution to the preferredbiooxidation pH of about 1.7 to 1.9. This is due to the bufferingcapacity of the bioleachate solution. Thus, when the treated anduntreated portions are recombined, the final pH of the entireconditioned bioleachate off solution should fall within the optimumrange for biooxidation. If not, minor adjustments can be made usingconcentrated acid or acid from the drain and wash circuit.

Even though only 70 to 85% of the off solution is actually treated inthis variation, sufficient inhibitory materials are removed from thisportion of the off solution so that when it is recombined with theremaining 15 to 30% of untreated off solution, the biooxidation rate ofthe entire conditioned off solution is improved substantially. Thus,this variation has the advantage of removing a substantial portion ofthe inhibitory materials from the bioleachate off solution, while usingabout the same amount of lime or limestone as is required to adjust thepH of the entire off solution back to the optimal range forbiooxidation.

Instead of applying the reconditioned bioleachate solution 22 to the topof heap 10, it can alternatively be applied to the top of heap 28. And,in order to maintain an appropriate level of activity within heap 28,bioleachate solution from tank 18 may be added on an as needed basisthrough line 32 to supplement the applications of reconditionedbioleachate solution 22 from heap 10.

Heap 28, like heap 10, is a heap of refractory sulfide ore particles.However, compared to heap 10, heap 28 is in a more advanced stage ofbiooxidation. Because heap 28 is in advanced stage of biooxidation, themajority of inhibitory materials have already been washed from heap 28.Thus, the effluent from this heap will be high in acidity (pH of about1), high in ferric ion concentration, and low in inhibitory components.The bioleachate off solution 30 from heap 28, therefore, may beadvantageously applied to heap 10, which is early in the biooxidationprocess, as the high ferric ion concentration and acidity coupled withthe low levels of inhibitory components will accelerate the initiationof biooxidation. Similarly, the reconditioned bioleachate off solution22 from heap 10 can be applied to the more fully biooxidized ore in heap28. As the more fully biooxidized ore in heap 28 is very acidic, theapplied bioleachate solution should not be as acidic as when applied toan ore that is less fully biooxidized. Further, because heap 28 containsless toxins, the rate of application of the bioleachate solution to thisheap can be reduced.

Preferably the pH of the on solution applied to heap 28 is in the rangeof about 2 to 3, instead of 1.7 to 1.9. Thus, less acid is required tolower the pH of the bioleachate off solution from heap 10 to the optimalbiooxidation range for heap 28 after it goes through lime or limestonesoftening in conditioning circuit 20. This means that a largerpercentage of the off solution may be treated in the lime or limestoneconditioning process.

To reiterate, the effluent acidity, ferric ion content and inhibitorymaterial content from different biooxidation heaps (corresponding toores that have been biooxidized to different extents) will be different.Any effluent that is high in the combined concentration of inhibitorymaterials should be treated to remove the inhibitory components. Appliedsolutions to the different heaps should be formed from mixtures of theavailable effluent solutions. Optimal mixtures can be made such that thesolution applied to a heap early in the biooxidation process is high inferric ion content, high in acidity and low in inhibitory components.Solutions applied to more fully biooxidized heaps can be lower inacidity. By using these optimal mixtures, initiation of biooxidationshould occur more quickly, and the amount of required neutralizationshould be less with a corresponding reduction in neutralizationassociated costs.

Ores having a high natural carbonate level may require an excessiveamount of acid to condition the ore to the desired pH level forbiooxidation. Waste generated acid from drain heap 12 or biooxidationheap 28 is preferably used in such circumstances to condition this ore.Waste acid should be used to the extent possible to lower the pH of theore so as to minimize the neutralization requirements of the bioleachateoff solution.

The solution management system described in connection with FIG. 1 canalso be used in conjunction with the "race track" heap biooxidationprocess illustrated in FIG. 2. Using the "race track" heap of FIG. 2permits the present invention to be practiced in a more restricted areawhen available space for biooxidation is limited.

According to FIG. 2 a circular "race track" type heap 40 is constantlybeing formed and reformed. Thus the heap expansion zone 42, whichrepresents the empty surface area is gradually moving around the circleformed by the "race track" heap 40. As new layers of ore 44 are beingadded at face 46, the agglomerated and preferably inoculated refractorysulfide particles approach the new face 46 of the freshly inoculated ore44. From a similarly moving removal front 43, the ore is taken away to acyanide leach heap as is known in the art. Likewise a moving wash front47 and its corresponding new wash front 48 illustrate the moving washsection 49 being treated to reduce acidity of the biooxidized ore in the"race track" heap 40. Biooxidation in heap 40 occurs in the biooxidationzone 41 between the moving wash front and the moving ore front.

In the examples below various aspects of the invention are furtheramplified and are intended as illustrations, but not limitations, of theinvention disclosed herein.

EXAMPLE 1

The Thiobacillus ferrooxidans strain used in the present example wasinitially started from a culture consisting of a consortium of ATCC14119, ATCC 19859, ATCC 23270, and ATCC 33020. However, the culturepresently used by the inventor is no longer pure. Over time, thisculture has become contaminated with wild strains. Accordingly, uponissuance of the present disclosure as a patent, a deposit of thisculture of Thiobacillus ferrooxidans will be made.

The present experiment was conducted to test the sensitivity of themodified strain of T. ferrooxidans to individual metals found in theeffluent from a column containing refractory sulfide ore particles froma Gilt Edge Mine ore sample, which is located in South Dakota. Thebioleachate effluent or off solution as a whole was found to beinhibitory to the biooxidation process.

The Gilt Edge Mine ore sample within the biooxidation column consistedof 8 Kg of -3/8 inch ore. An inductively coupled plasma emissionspectroscopy ("ICP") analysis of the bioleachate effluent solution fromthis ore was conducted to determine the concentration of the inhibitorymetals therein. Individual assays were then prepared with metalconcentrations identical to or slightly greater than those determined bythe ICP analysis of the effluent. The metals were added to the assays asmetal salts, primarily as sulfate salts. The individual metals were thentested for their ability to inhibit the oxidation of ferrous sulfate bythe T. ferrooxidans. To determine whether there was any inhibitoryeffect observed as a result of the concentration of the individualmetal, each assay was compared against a positive control.

Table 1 below lists the metals found to be present in the effluent, theconcentration of each metal in the effluent and in the individualassays, and whether an inhibitory effect was observed in the assay.

                  TABLE 1                                                         ______________________________________                                        METAL     ICP CONC.  ASSAY CONC. TOXICITY                                     ______________________________________                                        Al (in HCl)                                                                             290    ppm     300   ppm   yes                                      As        8-12   ppm     15    ppm   no                                       Cd        2.3    ppm     5     ppm   no                                       Co        1.5    ppm     2     ppm   no                                       Cr(Cr.sub.2 O.sub.7 /H.sub.2 O)                                                         0.4    ppm     1     ppm   no                                       Cu (as SO.sub.4)                                                                        680    ppm     700   ppm   no                                       Mn        17     ppm     20    ppm   Possibly                                 Ni        0.9    ppm     1     ppm   no                                       Sr        3.6    ppm     5     ppm   no                                       Zn (in HCl)                                                                             44     ppm     45    ppm   yes                                      ______________________________________                                    

Based on these results, further tests were run for aluminum and zinc inorder to determine minimum toxic levels for these metals. In addition,tellurium, molybdenum, and cooper were tested to determine the minimumconcentration at which these metals inhibit the biooxidation process.With regard to aluminum and zinc, the sulfate salts of these metals wereused for this test instead of the chloride salts. In these tests, noinhibitory effect was observed up to the maximum concentration tested,which was 5000 ppm for aluminum and 2000 ppm for zinc.

The chlorine salts of these--and other metals--showed slight inhibitionof iron oxidation at 0.2% Cl, which increased to full toxicity at about1% chloride ion. Thus, the inhibitory effect observed for aluminum andzinc in the first test was due to the chloride ion and not an inhibitoryeffect due to the metals themselves.

The inhibitory effect observed in the manganese assay was determined tobe the result of an unknown artifact. This was concluded from the factthat when the original effluent was conditioned by increasing its pH towithin the range of 5.0 to 6.0 the conditioned solution showed noinhibitory effect after readjustment of the pH for biooxidation. Indeed,the conditioned effluent performed as good or better than the positivecontrol. Yet, solubilized manganese is a very difficult metal to leachfrom the effluent. In fact, raising the pH of the effluent to a pH of6.0 will not precipitate manganese from the bioleachate solution. Thus,while the conditioned effluent still contained the same amount ofmanganese as it did before conditioning, after conditioning the effluentwas no longer inhibitory. This means that an artifact of some sortcaused the manganese assay to exhibit a minimal level of inhibition, andthe inhibition was not due to the manganese itself.

In sum, while the effluent solution from the Gilt Edge Mine ore wasinhibitory to the biooxidation process, the concentration of noindividual metal contained within the solution was inhibitory.

A summary of the individual metal toxicity tests done after the initialscreen are contained within Table 2.

                  TABLE 2                                                         ______________________________________                                                     HIGHEST CONC.                                                    METAL        TESTED       TOXICITY                                            ______________________________________                                        Al (as SO.sub.4)                                                                           5000 ppm     none                                                Zn (as SO.sub.4)                                                                           2000 ppm     none                                                Te (as TeO.sub.3)                                                                           500 ppm     none                                                Mo (as MoO.sub.4)                                                                           500 ppm     ≧50 ppm                                      Cu (as SO.sub.4)                                                                           5000 ppm     none                                                ______________________________________                                    

As seen from Table 2, Mo inhibited the rate of biooxidation of themodified strain of T. ferrooxidans when its concentration reached ≧50ppm in the test solutions.

In addition to the metals listed in Table 2, the inhibitory effect ofsodium was also tested using solutions containing various concentrationsof Na₂ SO₄. As a result of these tests, it was determined that Na wasnot toxic to the modified strain of T. ferrooxidans up to aconcentration of about 1.2M (Na).

EXAMPLE 2

Two samples of ore from the Gilt Edge Mine in South Dakota were preparedfor a bioleach shake flask test. The two samples of ore were ground for20 minutes in a ball mill. One sample was mixed with xanthate andfloated in a laboratory flotation cell to form a pyrite concentrate. Aportion of the pyrite concentrate was used to grow and acclimate alaboratory mixed culture of Thiobacillus ferrooxidans bacteria to thisore. The ore was inoculated with 5×10⁸ cells/ml in 0.5 strength 9K saltsmedium at a pH 2.2 and a pulp density of approximately 10% (5 g/50 ml.).The slightly higher pH was used to obtain better cell growth.

The composition of the standard 9K salts medium for T. ferrooxidans islisted below. The concentrations are provided in grams/liter. After themedium was prepared, the pH of the medium was adjusted to 2.2 using H₂SO₄.

    ______________________________________                                               (NH.sub.4)SO.sub.4                                                                      5                                                                   KCl       0.17                                                                K.sub.2 HPO.sub.4                                                                       0.083                                                               MgSO.sub.4.7H.sub.2 O                                                                   0.833                                                               Ca(NO.sub.3).4H.sub.2 O                                                                 0.024                                                        ______________________________________                                    

After 13 days of shaking at 250 rpm at 30° C. the bacteria solution wassplit into two fractions. One fraction was used to inoculate the wholeore sample that had been ground but not floated. The other fraction wasused to inoculate the pyrite float concentrate. The pulp densities were25% (250 g/1000 ml) for the whole ore and 10% (70 g/630 ml) for thepyrite concentrate. The starting pH was 1.9 for each sample and thestarting Eh was approximately 460 mV for each. The samples were shakenat 250 rpm and kept at 30° C. Prior to inoculation, a small sample ofeach was sent out for metal analysis. The percentages of both iron andcopper were used to calculate the total amount of iron and copper ineach experiment.

As the digestion proceeded, a small volume of liquid was removed andanalyzed for soluble iron and copper. This was used to determine thetotal amount of iron and copper in solution. The total amount of ironand copper in solution was then used to calculate the percentage of eachmetal leached as the reaction proceeded. The time in days that thesesamples were taken as well as the iron and copper levels in ppm andpercentage leached and the Eh and pH are listed in Table 3.

On day 28, both samples of ore were left to settle out to provide asupernatant that could be removed. The removed solution was replacedwith new 0.5×9K salts at a pH of about 1.8. The biooxidation continuedwith the fresh solution. Shortly after the removal of this solution,which was high in inhibitory metals, the Eh and rate of leachingincreased. This effect can be graphically seen in FIGS. 3 and 4, whichwere prepared from the data in Table 3.

After several weeks, the Eh and the rate of iron leaching again sloweddown. This indicated that toxic or inhibitory elements were againleached out into the bioleachate solution, and that the removal of theseelements were important for the rapid biooxidation of this or similarores.

                                      TABLE 3                                     __________________________________________________________________________           % Fe        % Cu                                                       DAYS   LEACHED/H                                                                            % Fe LEACHED/H                                                                            % Cu                                                                              Eh H                                                                              Eh cc                                       __________________________________________________________________________    1  0   10.230 1.590                                                                              64.190 39.890                                                                            485 448                                         2  3   13.520 1.860                                                                              72.140 55.540                                                                            486 455                                         3  6   14.770 1.940                                                                              80.060 66.100                                                                            470 445                                         4  10  15.090 2.130                                                                              93.130 76.740                                                                            469 449                                         5  13  17.610 2.230                                                                              102.350                                                                              81.800                                                                            473 453                                         6  18  16.000 2.280                                                                              101.310                                                                              81.810                                                                            489 469                                         7  21  17.060 2.680                                                                              104.510                                                                              85.880                                                                            474 467                                         8  24  16.960 2.710                                                                              103.790                                                                              84.090                                                                            448 436                                         9  28  16.050 2.720                                                                              101.080                                                                              85.410                                                                            437 420                                         10 31  16.660 4.150                                                                              102.020                                                                              85.550                                                                            461 545                                         11 35  19.830 8.970                                                                              104.820                                                                              86.740                                                                            545 567                                         12 39  23.240 11.96                                                                              104.850                                                                              87.170                                                                            611 556                                         13 42  25.770 14.46                                                                              106.080                                                                              87.710                                                                            555 544                                         14 45  26.290 15.10           527 514                                         15 49  28.300 15.71           519 501                                         16 52  29.110 16.19           535 522                                         17 63  32.310 15.27                                                                              107.020                                                                              88.740                                                                            531 527                                         __________________________________________________________________________     % Fe leached/H = % Fe leached from the whole ore                              % Fe = % Fe leached from the concentrate                                      % Cu leached/H = % Cu leached from the whole ore                              % Cu = % Cu leached from the concentrate                                      Eh H = The Eh of the solution coming of the whole ore sample.                 Eh cc = The Eh of the solution coming from the pyrite concentrate sample.

EXAMPLE 3

A second test was conducted with the ore used in Example 2 to simulate aheap biooxidation process. The sample provided by the Dakota MiningCorporation from the Gilt Edge Mine was crushed to minus 1/4 inchmaterial. In order to achieve good air flow, the fine material (passing30 mesh screen) was removed, which accounted for about 20% by weight ofthis 16 kg sample. A 7.8 Kg sample of the +30 mesh ore was mixed withsulfuric acid and 0.5×9K salts to wet the ore and lower the pH below2.0. The wet ore was placed into a 3 inch by 6 ft. column. Air wasintroduced into the bottom and liquid (0.2×9K salts, pH 1.8) andbacteria (˜10⁷ cell/g of ore) were applied to the top of the column. Thesolution coming off the bottom of the column was analyzed for iron andcopper, and the concentration of these metals in the solution samplesremoved from the column were used to calculate the total percentage ofiron and copper leached.

After 34 days, the solution coming off the column was re-applieddirectly to the top of the column without treatment. This was done tosee if the high ferric levels of recirculated solution would speed upthe leaching of pyrite. The leaching proceeded, but very slowly. After91 days, the column was changed back to a single pass system. Shortlyafter changing back to a single pass solution system, the leaching rateincreased. The effects of this change are shown in FIG. 5.

The pH, Eh, Fe concentration, and Fe leached for the various test timesof the effluent solution coming off the column are reported in Table 4.

                  TABLE 4                                                         ______________________________________                                        # OF                              % Fe                                        DAYS     pH     Eh (Volts) Fe (PPM)                                                                             LEACHED                                     ______________________________________                                         0       1.650  0.490      1414   0.570                                        2       1.570  0.490      1320   1.040                                        3       1.840  0.488      680    1.180                                        4       1.980  0.496      400    1.250                                        7       2.130  0.477      287    1.350                                        8       2.350  0.450      198    1.370                                        9       2.430  0.530      160    1.380                                       10       2.430  0.458      141    1.390                                       11       2.510  0.549      128    1.410                                       14       2.480  0.577      102    1.430                                       17       2.440  0.56       125    1.460                                       20       2.330  0.558      218    1.530                                       23       2.170  0.555      387    1.640                                       27       2.130  0.603      402    1.950                                       29       2.060  0.604      406    2.080                                       34       2.130  0.609      398    2.370                                       39       2.000  0.609      580                                                42       1.980  0.607      694                                                46       1.960  0.608      1052                                               49       1.870  0.602      1546                                               52       1.810  0.605      1838                                               57       1.900  0.604      2328                                               59       1.900  0.587      2586   3.510                                       64       1.830  0.591      3624                                               66       1.790  0.585      3930                                               67       1.710  0.587      3942                                               70       1.760  0.599      3778                                               73       1.760  0.587      4368                                               77       1.650  0.581      4984                                               81       1.660  0.577      5424                                               85       1.660  0.568      5912                                               91       1.620  0.565      4912                                               94       1.730  0.605      1722   5.400                                       100      1.770  0.620      1562   6.390                                       105      1.550  0.609      2124   7.120                                       12       1.880  0.684      2760   8.030                                       119      1.670  0.671      2916   9.810                                       129      1.470  0.679      3632   13.250                                      ______________________________________                                    

EXAMPLE 4

Another column test was performed with the Gilt Edge Mine ore of Example2. In this example, the -10 mesh material was removed from the ore, andthe ore was only crushed to -3/8 inch. The ore was prepared as beforeand placed into a 3 inch by 6 ft. column with air from the bottom andliquid from the top as in Example 3. Further, only fresh solution wasintroduced from the top of the column. The rate of leaching wasdetermined by the amount of metal removed as before. The percent of ironleached was plotted in FIG. 6 for comparison to FIG. 5, which representsthe whole ore column in Example 3. From this comparison, one can seethat the recirculation of leach solution was inhibitory to the rate ofbiooxidation.

The pH, Eh, and % Fe leached at the various test times of the effluentsolution are reported in Table 5.

                  TABLE 5                                                         ______________________________________                                        TIME IN DAYS                                                                              pH        Eh (Volts)                                                                             % Fe Leached                                   ______________________________________                                         5          1.630     0.528    0.500                                          16          1.890     0.560    0.650                                          23          1.890     0.672    1.290                                          30          1.830     0.657    2.280                                          40          1.680     0.672    4.890                                          45          1.780     0.684    5.770                                          55          1.770     0.684    8.740                                          65          1.800     0.705    10.630                                         75          1.520     0.719    12.650                                         86          1.630     0.705    14.600                                         95          1.830     0.697    16.020                                         104         1.780     0.705    17.470                                         114         1.930     0.703    18.070                                         123         2.070     0.678    18.570                                         ______________________________________                                    

EXAMPLE 5

Effluent from a heap biooxidation field test utilizing 4,750 dry shorttons of refractory sulfide ore from the Gilt Edge Mine, near Deadwood,S.D., was collected 63 days into the test. This ore containedapproximately 5.5% sulfides as sulfur and 1.78 g Au/ton of ore. The rateof biooxidation as measured by changes in the sulfate concentration inthe effluent solution was lower than desired due to the concentration ofinhibitory components in the effluent solution. The effluent solutionwas being recirculated and applied to the test heap withoutconditioning.

Samples of the effluent were adjusted to various pH's to ascertain theminimum pH for removal of the toxic components. The effluent had a pH of2.0. Aliquots of the effluent were adjusted to 3.0, 3.5, 4.0, 4.5, 5.0,and 5.5, with lime, in separate tubes. The supernatant from thesealiquots was collected for inhibition testing with Thiobacillusferrooxidans. Ferrous sulfate heptahydrate was added to each supernatantat a concentration of 20 grams per liter, and the pH was adjusted to1.7-1.9 with sulfuric acid. The electrochemical potential (Eh) of eachsolution was recorded (Time 0), the solutions were inoculated with T.ferrooxidans, and were shaken at 34° C. overnight. The change in Eh wasmonitored as an indication of biooxidation of the ferrous iron. Theresults are tabulated in Table 6 below.

The highest pH supernatant, at 5.5, consistently produced the highest Eh(highest ferric concentration), equaling or surpassing the positivecontrol. The lower pH supernatants showed a smaller increase in thesample Eh, indicative of a lower level of bacterial activity, suggestingincomplete removal of the toxins from the solution as illustrated inFIG. 7. Adjusting the effluent solution to a pH of 5.5 or higher,however, was necessary and sufficient to precipitate all the inhibitorycomponents in the effluent solution for this ore.

An additional sample of effluent solution was adjusted to a pH 6, withlime, producing a sludge. Without removing the sludge, the pH wasreadjusted to 1.8. The resulting liquid from this mixture was tested fortoxicity to T. ferrooxidans. The toxicity of the readjusted mixture wasthe same as that of the original effluent solution, indicating that allthe toxic components precipitated into the sludge at a pH >5.5 and werethen resolubilized during adjustment back to pH 1.8 as illustrated inFIG. 8. This example shows the need for removing the metal precipitatesduring lime or limestone softening before the pH of the solutions isreadjusted for biooxidation.

                  TABLE 6                                                         ______________________________________                                                       Eh AT START                                                                              Eh AT 24 HRS.                                       SAMPLE         (mV)       (mV)                                                ______________________________________                                        UNADJUSTED OFF 450        589                                                 SOLUTION                                                                      pH 3.0         408        638                                                 pH 3.5         385        644                                                 pH 4.0         375        642                                                 pH 4.5         356        592                                                 pH 5.0         346        585                                                 pH 5.5         337        647                                                 positive       339        611                                                 control                                                                       ______________________________________                                    

EXAMPLE 6

A heap biooxidation field test utilizing 4,750 dry short tons ofrefractory sulfide ore from the Gilt Edge Mine, near Deadwood, S.D. wasran. The test ore contained 5.5% sulfides as sulfur and 1.78 grams goldper ton of ore. Extraction by conventional bottle roll cyanidation testsyielded a recovery of 56%.

During the heap biooxidation test, the extent of biooxidation wasdetermined by examining the concentration of sulfate ions in theeffluent solution. FIG. 9 plots the extent of biooxidation as thepercent of sulfides oxidized against the time into the test. Except asindicated below, the effluent solution was recycled to the top of theheap without conditioning.

Initially, the field test demonstrated a rapid rate of biooxidation,which quickly slowed down. The slowing down corresponds with an increasein the observed solubilized metals including copper in the effluentsolution. The initial biooxidation rate was 0.133% per day (day 3through 13). During this time, the water inventory was being establishedin the biooxidation heap and fresh water was being applied. With thiscontinuous addition of fresh water the inhibiting components were keptdilute and did not effect the rate of biooxidation. After fresh waterwas no longer being added to the surface of the heap, at day 13, thebiooxidation rate slowed.

At approximately day 23, a batch neutralization of a portion of the heapeffluent was conducted by raising the pH of the solution to above 5.5,removing the solid precipitates and then lowering the pH of the solutionback to approximately 1.8. After this batch neutralization wasperformed, the biooxidation rate increased until about day 35 when itleveled off due to increased concentrations of solubilized metals. Onday 38 approximately 10% of the effluent solution was pH adjusted toover 5.5, removing the inhibitory components. The rate of biooxidationafter removal of these inhibitory components was improved until day 42when the concentration of inhibitory metals suppressed the biooxidationrate. A small inflection point is also observed at about day 51 in FIG.9. This was due to the dilution of the inhibitory components in theeffluent with fresh water.

EXAMPLE 7

A three column system was constructed to simulate a large scale heapbiooxidation process. Three batches of 3/8 inch crushed and agglomeratedore that had been spayed with bacteria, each of about 8 kg, were placedinto three different columns having a diameter of 3 inches and a heightof 6 feet. Only the first of the three columns was provided with airflow. The other two columns were closed to simulate the air limitationthat may occur in a large heap. The test was started by applying fresh0.2×9K salts having a pH of 1.8 to the top of the first column at therate of about 200 ml/day or 0.0007 gal/ft² /min.

After about three days, the solution eluting off the bottom of the firstcolumn was pumped onto the second column without any treatment. Afteranother three days, the solution eluting from the second column waspumped onto the third column. The solution eluting from the third columnwas collected until the volume was over one liter.

After 15 days of operation, the first liter was treated with powderedlimestone to raise the pH to over 5.5. The precipitate was removed byfiltration. The treated solution was then mixed back with untreated offsolution at the rate of 85% treated solution (over pH 5.5) and 15%untreated solution (pH 1.6 or lower). If the resulting mixture was abovepH 2.0, then sulfuric acid was used to adjust the pH back down to 2.0 orbelow.

After 18 days from the start of the operation, the mixture of treatedand untreated off solution was used to replace the addition of fresh pH1.8 solution to the first column. With time, the volume of liquiddecreased to the point where fresh solution had to be added to thesystem to make up for water removed in the precipitation process andlost to evaporation. This system was meant to mimic a field operationwhere as much water as possible must be recycled. This system also madeuse of the acid generated from the biooxidation process to adjust the pHback down to below 2 after the lime or limestone treatment.

The first recycled solution took about 10 days to move through the threecolumns. A small drop in the rate of biooxidation was noted between theperiod between day 29 and day 31. The recycled solution was first usedon day 18 and took about 10 days to pass through all three columns. Thismeant that it would be day 28 when a change would show up in thebiooxidation rate. The rate of iron leaching dropped from 0.132%/day to0.103%/day. This change was considered small enough that the system wasworking at removing the build-up of toxic metals in the bioleachate offsolution. Further, the reuse of most of the water in the system wasconsidered of sufficient economic value to justify the small drop in thebiooxidation rate over the rate obtainable by using fresh solution only.

The pH, Eh, Fe concentration, and % Fe leached are reported in Table 7below for various times in the biooxidation process. These values weredetermined by testing the heap effluent at the times indicated in thetable.

                  TABLE 7                                                         ______________________________________                                                          Eh               % Fe                                       # OF DAYS  pH     (Volts)   Fe (PPM)                                                                             LEACHED                                    ______________________________________                                        15         1.820  0.576     6928   0.470                                      18         1.770  0.590     5116   0.700                                      24         1.680  0.614     8640   1.320                                      29         1.480  0.540     10136  1.980                                      31         1.490  0.507     7712   2.290                                      39         1.600  0.542     7312   2.960                                      ______________________________________                                    

EXAMPLE 8

This test was conducted to determine acute and chronic toxicity in themodified T. ferrooxidans strain used by the inventors. Dilutions of aneffluent produced in the early stages of a column biooxidation weretested using the microtiter plate assay for their ability to inhibit theiron oxidizing activity of T. ferrooxidans ("acute" toxicity). As seenin FIG. 10, when compared against a positive control sample of T.ferrooxidans in a ferrous sulfate medium, only the 1:10 and 1:5dilutions allowed adequate iron oxidation. The lower dilutions (1:2 andundiluted) inhibited most of the iron oxidizing activity of the cells.

The "chronic" toxicity of the effluent dilutions was tested next withthe spectrophotometric activity assay. Cells from the microtiter plateassay were collected, washed, and resuspended in a 0.2 strength 9K saltsmedium to which 2 mg/ml ferrous sulfate was added. The concentration offerric iron produced was measured over time and plotted in FIG. 11. Therates of activity were calculated from the resulting curves in FIG. 11.These values are reported in Table 8. The activities of the cells infresh medium were similar to the activities of the cells in theeffluent. Similar results were obtained when the spectrophotometricactivity assay was repeated after overnight incubation of the cells inferrous sulfate medium. The results suggest that the effluent caninhibit the cells for some time after they are removed from directcontact with the inhibited effluent.

                  TABLE 8                                                         ______________________________________                                        SAMPLE   FERROUS-FERRIC CONVERSION RATE (mg/min)                              ______________________________________                                        Positive control                                                                       1.72                                                                 1:10 dil.                                                                              1.92                                                                 1:5 dil. 1.94                                                                 1:2 dil. 1.10                                                                 neat (undiluted)                                                                       .06                                                                  ______________________________________                                    

EXAMPLE 9

As discussed above, one method for treating a biooxidation effluentwhich is inhibitory to T. ferrooxidans is to raise the pH enough toprecipitate the inhibitory substances and then readjust the pH to theoptimum range for biooxidation after separating out the precipitates.This procedure was performed with the effluent from Example 8, and theresulting supernatant solutions were tested for their ability to inhibitthe bacteria in the microtiter plate assay. The results, shown in FIG.12, indicate that the inhibition is removed at a pH of between 5 and 6.Partial activity is seen in pH 5 supernatant and full activity isrestored by pH 6.

Although the invention has been described with reference to preferredembodiments and specific examples, it will readily be appreciated bythose of ordinary skill in the art that many modifications and adaptionsof the invention are possible without departure from the spirit andscope of the invention as claimed hereinafter.

We claim:
 1. A method for improving the heap biooxidation rate ofrefractory sulfide ore particles that are at least partially biooxidizedusing a recycled bioleachate off solution, the process comprising:a.biooxidizing a heap comprised of refractory sulfide ore particles with abioleachate solution and thereby producing a bioleachate off solutionthat includes a plurality of inhibitory materials dissolved therein,wherein the concentration of each individual inhibitory material in thebioleachate off solution is below its individual inhibitoryconcentration and the combined concentration of at least two of theinhibitory materials is sufficient to inhibit the biooxidation rate ofthe refractory sulfide ore particles in the ore; b. collecting thebioleachate off solution from the heap; c. conditioning the bioleachateoff solution to reduce the inhibitory effect caused by the combinedconcentration of the at least two inhibitory materials; d. recycling thebioleachate off solution to the heap;and e. biooxidizing the refractorysulfide ore particles in the heap with the recycled bioleachate offsolution.
 2. A method for improving the heap biooxidation rate ofrefractory sulfide ore particles according to claim 1, wherein themethod of conditioning the bioleachate off solution is at least oneselected from the group consisting of lime softening, limestonesoftening, ion exchange, electrodeposition, iron cementation, andreverse osmosis.
 3. A method for improving the heap biooxidation rate ofrefractory sulfide ore particles according to claim 1, wherein themethod of conditioning the bioleachate off solution is at least oneselected from the group consisting of lime softening and limestonesoftening.
 4. A method for improving the heap biooxidation rate ofrefractory sulfide ore particles according to claim 3, wherein the pH ofthe bioleachate off solution is raised to a pH of at least 5.0 duringthe conditioning step.
 5. A method for improving the heap biooxidationrate of refractory sulfide ore particles according to claim 3, whereinthe pH of the bioleachate off solution is raised to a pH in the range of5.0 to 6.0 during the conditioning step.
 6. A method for improving theheap biooxidation rate of refractory sulfide ore particles according toclaim 3, wherein the pH of the bioleachate off solution is raised to apH in the range of 5.5 to 6.0 during the conditioning step.
 7. A methodfor improving the heap biooxidation rate of refractory sulfide oreparticles that are at least partially biooxidized using a recycledbioleachate off solution, the process comprising:a. biooxidizing a heapcomprised of refractory sulfide ore particles with a bioleachatesolution and thereby producing a bioleachate off solution that includesa plurality of inhibitory materials dissolved therein; b. collecting thebioleachate off solution from the heap; c. raising the pH of thebioleachate off solution to a pH greater than 5.0 and thereby forming aprecipitate; d. removing the precipitate from the bioleachate offsolution; e. adjusting the pH of the bioleachate off solution to a pHsuitable for biooxidation following the removal of the precipitate; f.recycling the bioleachate off solution to the heap; and g. biooxidizingthe refractory sulfide ore particles in the heap with the recycledbioleachate off solution.
 8. A method for improving the heapbiooxidation rate of refractory sulfide ore particles according to claim7, wherein the pH of the bioleachate off solution is raised to a pH inthe range of 5.0 to 6.0.
 9. A method for improving the heap biooxidationrate of refractory sulfide ore particles according to claim 7, whereinthe pH of the bioleachate off solution is raised to a pH in the range of5.5 to 6.0.
 10. A method for improving the heap biooxidation rate ofrefractory sulfide ore particles according to claim 7, wherein steps c.,d., and e. are performed only after the bioleachate off solution becomesinhibitory due to the presence of a combination of inhibitory materials.11. A method for improving the heap biooxidation rate of refractorysulfide ore particles according to claim 7, wherein the pH of thebioleachate off solution is raised to a pH of at least 5.5.
 12. A methodfor improving the heap biooxidation rate of refractory sulfide oreparticles according to claim 7, wherein the pH of the bioleachate offsolution is raised to a pH of at least 6.0.
 13. A method for improvingthe heap biooxidation rate of refractory sulfide ore particles as in oneof claims 7-12, wherein the pH of the bioleachate off solution isadjusted to a range of 1.2 to 2.6 following the removal of theprecipitate.
 14. A method for improving the heap biooxidation rate ofrefractory sulfide ore particles as in one of claims 7-12, wherein thepH of the bioleachate off solution is adjusted to a range of 1.7 to 1.9following the removal of the precipitate.
 15. A method for improving theheap biooxidation rate of refractory sulfide ore particles according toclaim 7, wherein the bioleachate off solution is recycled to the heapbya. agglomerating refractory sulfide ore particles with the bioleachateoff solution; and b. adding the agglomerated refractory sulfide oreparticles to the heap.
 16. A method for improving the heap biooxidationrate of refractory sulfide ore particles according to claim 3, whereinthe pH of the bioleachate off solution is raised to a pH of at least 5.5during the conditioning step.
 17. A method for improving the heapbiooxidation rate of refractory sulfide ore particles according to claim3, wherein the pH of the bioleachate off solution is raised to a pH ofat least 6.0 during the conditioning step.
 18. A method for at leastpartially biooxidizing a heap comprised of refractory sulfide oreparticles using a bioleachate off solution that includes a plurality ofinhibitory materials dissolved therein, wherein the concentration ofeach individual inhibitory material in the bioleachate off solution isbelow its individual inhibitory concentration, yet the combinedconcentration of at least two of the inhibitory materials is sufficientto inhibit the biooxidation rate of the refractory sulfide ore particlesin the ore, the process comprising:a. conditioning the bioleachate offsolution to reduce the inhibitory effect caused by the combinedconcentration of the at least two inhibitory materials; b. recycling thebioleachate off solution to the heap; and c. biooxidizing the refractorysulfide ore particles in the heap with the bioleachate off solution. 19.A method according to claim 18, wherein conditioning the bioleachate offsolution comprises:a. raising the pH of the bioleachate off solution toa pH of at least 5.0 to form a precipitate, b. removing the precipitatefrom the bioleachate off solution; and c. adjusting the pH of thebioleachate off solution to a pH suitable for biooxidation following theremoval of the precipitate.
 20. A method according to claim 19, whereinthe pH of the bioleachate off solution is raised to a pH of at least5.5.
 21. A method according to claim 19, wherein the pH of thebioleachate off solution is raised to a pH of at least 6.0.
 22. A methodaccording to claim 19, wherein the pH of the bioleachate off solution israised to a pH in the range of 5.0 to 6.0.
 23. A method according toclaim 19, wherein the pH of the bioleachate off solution is raised to apH in the range of 5.5 to 6.0.
 24. A method as in one of claims 19-23,wherein the pH of the bioleachate off solution is adjusted to a range of1.2 to 2.6 following the removal of the precipitate.
 25. A method as inone of claims 19-23, wherein the pH of the bioleachate off solution isadjusted to a range of 1.7 to 1.9 following the removal of theprecipitate.
 26. A method as in one of claims 19-23, wherein the pH ofthe bioleachate off solution is adjusted to a range of 2 to 3 followingthe removal of the precipitate.
 27. A method according to claim 18,wherein the bioleachate off solution is obtained from the heap.
 28. Amethod according to claim 18, wherein the bioleachate off solution isobtained from a second heap comprised of refractory sulfide oreparticles that is being biooxidized.
 29. A method according to claim 18,wherein the bioleachate off solution is recycled to the heap bya.agglomerating refractory sulfide ore particles with the bioleachate offsolution; and b. adding the agglomerated refractory sulfide oreparticles to the heap.
 30. A method for at least partially biooxidizinga heap comprised of refractory sulfide ore particles using a bioleachateoff solution that includes a plurality of inhibitory materials dissolvedtherein, the process comprising:a. raising the pH of the bioleachate offsolution to a pH greater than 5.0 and thereby forming a precipitate; b.removing the precipitate from the bioleachate off solution; c. adjustingthe pH of the bioleachate off solution to a pH suitable for biooxidationfollowing the removal of the precipitate; d. recycling the bioleachateoff solution to the heap; and e. biooxidizing the refractory sulfide oreparticles in the heap with the bioleachate off solution.
 31. A methodaccording to claim 30, wherein the pH of the bioleachate off solution israised to a pH of at least 5.5.
 32. A method according to claim 30,wherein the pH of the bioleachate off solution is raised to a pH of atleast 6.0.
 33. A method according to claim 30, wherein the pH of thebioleachate off solution is raised to a pH in the range of 5.0 to 6.0.34. A method according to claim 30, wherein the pH of the bioleachateoff solution is raised to a pH in the range of 5.5 to 6.0.
 35. A methodas in one of claims 30-34, wherein the pH of the bioleachate offsolution is adjusted to the range of 1.2 to 2.6 following the removal ofthe precipitate.
 36. A method as in one of claims 30-34, wherein the pHof the bioleachate off solution is adjusted to the range of 1.7 to 1.9following the removal of the precipitate.
 37. A method as in one ofclaims 30-34, wherein the pH of the bioleachate off solution is adjustedto the range of 2 to 3 following the removal of the precipitate.
 38. Amethod according to claim 30, wherein the bioleachate off solution isobtained from the heap.
 39. A method according to claim 30, wherein thebioleachate off solution is obtained from a second heap comprised ofrefractory sulfide ore particles that is being biooxidized.
 40. A methodaccording to claim 30, wherein the bioleachate off solution isinhibitory to the biooxidation rate of the refractory sulfide oreparticles due to the presence of a combination of at least twoinhibitory materials.
 41. A method according to claim 30, wherein thebioleachate off solution is recycled to the heap bya. agglomeratingrefractory sulfide ore particles with the bioleachate off solution; andb. adding the agglomerated refractory sulfide ore particles to the heap.42. A method for improving the heap biooxidation rate of refractorysulfide ore particles according to claim 7, wherein the bioleachate offsolution is inhibitory to the biooxidation rate of the refractorysulfide ore particles due to the presence of a combination of at leasttwo inhibitory materials.
 43. A method for conditioning a bioleachateoff solution that has been used to biooxidize refractory sulfide ore,wherein the bioleachate off solution includes a plurality of inhibitorymaterials dissolved therein, the process comprising:a. raising the pH ofthe bioleachate off solution to a pH greater than 5.0 and therebyforming a precipitate; b. removing the precipitate from the bioleachateoff solution and c. adjusting the pH of the bioleachete off solution toa pH suitable for biooxidation following removal of the precipitate. 44.A method according to claim 43, wherein the pH of the bioleachate offsolution is raised to a pH of at least 5.5.
 45. A method according toclaim 43, wherein the pH of the bioleachate off solution is raised to apH of at least 6.0.
 46. A method according to claim 43, wherein the pHof the bioleachate off solution is raised to a pH in the range of 5.0 to6.0.
 47. A method according to claim 43, wherein the pH of thebioleachate off solution is raised to a pH in the range of 5.5 to 6.0.48. A method as in one of claims 43-47, wherein the pH of thebioleachate off solution is adjusted to the range of 1.2 to 2.6following the removal of the precipitate.
 49. A method as in one ofclaims 43-47, wherein the pH of the bioleachate off solution is adjustedto the range of 1.7 to 1.9 following the removal of the precipitate. 50.A method as in one of claims 43-47, wherein the pH of the bioleachateoff solution is adjusted to the range of 2 to 3 following the removal ofthe precipitate.
 51. A method according to claim 43, wherein thebioleachate off solution is obtained from a heap comprising ofrefractory sulfide ore particles that is being biooxidized.
 52. A methodaccording to claim 43, wherein the bioleachate off solution isinhibitory to the biooxidation rate of refractory sulfide ore particles.